Field of the Invention
The present invention relates to a printing method and apparatus and, more particularly, to a printing method and apparatus which print multilevel grayscale images.
Among printers based on various printing schemes, some printers are designed to form texts and images on printing media by making printing materials adhere to the printing media. Of the printers based on such printing schemes, an ink-jet printing apparatus is a typical one. Recently, with advances in the performance of ink-jet printing apparatuses, images have been printed as well as texts.
A typical ink-jet printing apparatus uses an array of a plurality of orifices (nozzles) capable of discharging inks having the same color and density. Such arrays of nozzles are generally arranged for inks having the same color and different densities or inks having different colors, respectively. Some printing apparatuses can discharge inks having the same color and density while changing the discharge amount in several steps.
While a head having these nozzles is moved relative to a printing medium, ink is discharged from the nozzles, thereby printing an image.
As methods of moving a head relative to a printing medium, the following are practiced:
(i) A so-called swath printing scheme, in which nozzles are arranged substantially parallel in the X direction. While a printing medium is at rest, the printhead is moved in a direction (Y direction) perpendicular to the X direction, and printing is performed during this period. Thereafter, the printing medium is intermittently moved by a predetermined distance in the X direction. The printhead is then moved again in the Y direction. Subsequently, this operation is repeated to print.
(ii) A so-called full multi-printing method, in which nozzles are fixed to cover the entire width of a printing medium in the Y direction. Printing is performed while the printing medium is moved at a constant speed in the X direction.
When images are printed by these methods, a pixel is defined as a unit of an image. A pixel is not necessarily formed by one dot (a portion formed on a printing medium by discharging ink from one nozzle once) and may be formed by a plurality of dots. When each pixel is to be formed by a plurality of dots, dots may be overlaid and printed on substantially the same point or may be printed on adjacent points. In either case, overlaying operation is determined in accordance with a predetermined rule. Image data to be printed is subjected to enlargement interpolation, reduction, or the like by an image processing means to have an image size conforming to a printing apparatus. The color to be printed and its density are determined for each pixel in accordance with predetermined rules. Printing is then executed in accordance with this determination. As described above, since one pixel may be constituted by a plurality of dots, dots do not necessarily have the same density, and inks having different densities can be selected. If a head capable of changing the discharge amount is used, the discharge amount, i.e., the ink amount of a dot, may be changed as needed. Alternatively, these methods may be combined.
When an image is to be printed, halftoning such as dithering or error diffusion is used as a method of faithfully reproducing the gradation of image data. In dithering or error diffusion, by increasing the number of gray levels of one pixel, a larger number of gray levels can be expressed. Such a printing method is disclosed in detail, for example, in Japanese Patent Laid-Open No. 10-324002.
More specifically, nozzles capable of discharging inks having different densities are prepared for one color, and printing is selectively performed a plurality of number of times (to be referred to as overlaying hereinafter) for one pixel by using these nozzles within a predetermined limit, thereby increasing the number of gray levels or densities (printing OD values) that can be expressed on this pixel. Assume that nozzles capable of discharging inks having six different densities are prepared, and overlaying is to be performed four times or less with respect to one pixel based on 600 dpi. In this case, 50 gray levels or more can be expressed. If one pixel is constituted by 2xc3x972 adjacent points and is to be formed by a total of 16 times or less of overlaying/printing, 200 gray levels or more can be expressed. Gradation may be expressed by changing the amount of ink discharged from each ink and changing the ink amount of each dot instead of preparing nozzles capable of discharging inks having different densities. Alternatively, gradation may be expressed by combining these methods.
In these cases, a rule that makes the density (desired OD value) of a pixel to be expressed correspond to an ink overlaying/printing method is determined in advance, and actual printing, i.e., which nozzles are used and when inks are discharged, is determined in accordance with this rule. Printing is then actually performed by a printing control means in accordance with this determination.
For example, the printing OD value of each pixel printed by using the respective inks is measured in advance, and a printing OD value obtained by overlaying is determined by this measurement value, thereby preparing a table in which the printing OD values of pixels corresponding to the respective overlay patterns are written. An overlay pattern corresponding to a printing OD value near a desired OD value of a pixel to be printed is selected. In error diffusion processing, the difference between the desired OD value of the pixel to be printed and the corresponding printing OD value in the table is obtained and is distributed as errors to adjacent pixels.
There are various kinds of images, and hence various characteristics are required for printers depending on applications and purposes. In designing various printers in accordance with the application purposes, it is preferable that printing characteristics be freely designed.
As an example of an image for which special printing characteristics are required, a medical image will be described below.
In some fields, e.g., the field of medical images, many monochrome images printed in monochrome are still used for the following reason. A monochrome image exhibits high human eye density resolution. Therefore, in a field in which high density resolution is required, the amount of information that can be recognized by a human is higher in a monochrome image than in a color image. It is known that a transmission type printing medium increases the human eye density resolution as compared with a reflection type printing medium. In general, the human eye density resolution with respect to a color image is about 8 bits, whereas that with respect to a monochrome transmission image is 10 to 11 bits. A medical X-ray photograph or CT/MRI image printed on a transmission medium is actually read up to the human eye resolution limit to provide information for diagnosis. As a printer for printing such a high-quality monochrome image, a laser imager is available, which irradiates a silver halide film with a laser beam modulated in accordance with an image signal, and developing the film, thereby obtaining an image on the film. In such a laser imager, an image is often printed with a density resolution of 12 bits in consideration of a certain margin. However, such a laser image is expensive. In addition, wet type developing processing is required, and hence problems arise in terms of waste liquid disposal, cumbersome maintenance, and the like. Although a dry silver halide type laser imager which develops by heating is available, the image quality is inferior to that in the wet type.
An apparatus based on the ink-jet scheme capable of expressing 50 density gray levels or more in 600 dpi is disclosed in Japanese Patent Laid-Open No. 10-324002, which can print a 256-level grayscale image by further performing error diffusion processing. This reference has exemplified 256-level grayscale printing. If input image data is 4096-level grayscale data instead of 256-level grayscale data, 4096-level grayscale printing can be performed.
Table 1 shows the inks used by the printhead in this reference. As shown in Table 1, six types of inks are used, i.e., inks A, B, C, D, E, and F in descending order of density. Table 1 also shows the dye densities (%) and transmission densities of the respective inks A to F. Note that each ink is made of a dye and solvent. The solvent contains various additives such as a surfactant and humectant. These additives are used to control the discharge characteristics of ink from a printhead and the absorption characteristics of ink on a printing medium.
If these inks are used and the maximum number of types of inks that can be ejected on one pixel is set to 4, the number of gray levels that can be expressed by one pixel is 6+6C2+6C3+6C4+1=57. In Table 1, inks having dye density that inhibit a combination of inks having the same density are set. The density ratios of four types of ink dots on the low-density side are 1:2:4:8 in ascending order of density. An image is output by using 53 gray levels of these 57 gray levels. That is, as described above, input image data (4096 gray levels) is converted into 53-base image data to output an image. FIG. 19 shows the types of inks and combinations thereof which are used to express the respective gray levels (53 gray levels). Referring to FIG. 19, the No. column indicates each gray level. Each portion indicated by the symbol xe2x80x9cxe2x80x9d in FIG. 19 indicates an ink combination that is not used to make the density level difference at a low-density portion become smaller than that at a high-density portion. In each of the ink A to F columns, xe2x80x9cOxe2x80x9d indicates that the corresponding ink is discharged from the printhead, and xe2x80x9cxxe2x80x9d indicates that the ink is not discharged from the printhead. In addition, the dl[i] (i=0 to 52: integers) column indicates the ink density level that expresses each gray level. The th[i] (i=1 to 52: integers) indicates a threshold for determining input image data as data corresponding to a specific one of 53 gray levels. Note that a threshold is generally determined as an ink density level at a midpoint between an ink density level dl[kxe2x88x921] and an ink density level dl[k].
In this case, a combination of ink types that indicates each gray level is combination data, and the ink density level determined on the basis of the combination data is ink density data.
A multilevel error diffusion processing unit performs multilevel error diffusion processing to convert input image data (4096 gray levels) into base-53 data by using 53 ink density levels (dl[0] to dl[52]) and 52 thresholds (th[1] to th[52]). As described above, in the multilevel error diffusion processing disclosed in the reference, a plurality of thresholds, 52 values in this case, are set to convert input image data into multilevel data. In this point, this error diffusion processing greatly differs from general error diffusion processing. In this case, input image data is converted into multilevel data by using multilevel error diffusion processing. However, this operation is not limited to this method. For example, input image data may be converted into multilevel data by using another multilevel conversion method such as the multilevel average density retention method, multilevel dither matrix method, or submatrix method.
A procedure for printing control on the ink-jet printing apparatus disclosed in the above reference will be described next with reference to the flow chart of FIG. 20.
FIG. 20 is a flow chart showing the procedure for printing control on the ink-jet printing apparatus disclosed in the above reference.
In step S1, data associated with inks to be used by a printhead, including ink density data and combination data, is stored in an ink density data/combination data unit.
In step S2, input image data is input, and multilevel error diffusion processing is performed for each pixel indicated by the input image data.
Multilevel error diffusion processing will be described in detail below with reference to FIGS. 21A and 21B.
FIGS. 21A and 21B are views showing the arrangement of input image data in the above reference and the arrangement of base-53 image data obtained after multilevel error diffusion processing. That is, FIGS. 21A and 21B show part of the arrangement of pixels in 4096 density data (0 (black) to 4095 (transparent)) of the respective pixels of the input image data.
Referring to FIG. 21A, f(i, j) represents the 4096-density data level of a pixel of interest (i, j) to be converted into multilevel (base-53) data. Each of pixels f(ixe2x88x922, jxe2x88x921) to (ixe2x88x921, j) above the dashed line have already undergone conversion to multilevel (base-53) data, and B(i, j) represents density data (53 values xe2x80x9c0xe2x80x9d, xe2x80x9c137.6xe2x80x9d, . . . , xe2x80x9c4011.2xe2x80x9d, xe2x80x9c4080xe2x80x9d) obtained by converting the pixel of interest (i, j) into multilevel (base-53) data. After conversion of the pixel of interest (i, j) into multilevel (base-53) data, conversion to multilevel (base-53) data is sequentially performed for f(i, j+1), f(i, j+2), . . . .
First of all, the 4096-density data level f(i, j) of the pixel of interest (i, j) is compared with a threshold th[k] by
th[k]xe2x89xa6f(i, j) less than th[k+1]xe2x80x83xe2x80x83(1)
B(i, j)=dl[k]xe2x80x83xe2x80x83(2)
A value k that satisfies relation (1) is then obtained, and the density data B(i, j) after conversion of the pixel of interest (i, j) into multilevel (base-53) data is determined according to equation (2).
Subsequently, an error err between the density data B(i, j) determined by the above conversion to multilevel data and the 4096-density data level f(i, j) before the conversion to multilevel data is calculated using the error diffusion matrix shown in FIG. 21B by:
err=f(i, j)xe2x88x92dl[k]xe2x80x83xe2x80x83(3)
The calculated error err is then diffused to other pixels by:
f(x, y)=f(x, y)+err xc3x97M(xxe2x88x92i, yxe2x88x92j)/31xe2x80x83xe2x80x83(4)
As described above, the error err is diffused to each pixel in accordance with an error diffusion matrix like the one shown in FIG. 21B. Thereafter, conversion to multilevel (base-53) data is performed in the same manner as described above by using a value f(i, j) containing the diffused error.
In step S3, ink discharge control data corresponding to the printhead is generated on the basis of the combination data shown in FIG. 19 which corresponds to the density data B(i, j) obtained by a data distribution unit by the conversion to multilevel (base-53) data. If, for example, the density data B(i, j)=1036.8, ink discharge control data is generated to discharge the inks A, C, D, and F.
In step S4, a printhead/paper feed control unit controls driving of the printhead and conveyance of the printing medium in accordance with the ink discharge control data, thereby forming a grayscale image.
According to the above reference, six ink-jet heads (256 nozzle match head) corresponding to 600 dpi are used to output a medical grayscale image (transmission).
As described above, according to the prior art disclosed in the above reference, a printhead capable of discharging a plurality of types of multi-density inks in the conveying direction (sub-scanning direction) of a printing medium is prepared, and at least one ink dot for forming an image is discharged from the head in forming an image. This makes it possible to increase the number of gray levels of an image to be printed by using an arrangement similar to that of the conventional printhead without forming any new printhead capable of discharging many types of inks. That is, a good grayscale image with a large number of gray levels can be obtained without at least greatly increasing the cost by, e.g., forming a new printhead.
When an X-ray image was actually printed by this scheme (4096 gray levels), high image quality was obtained. Depending on the types of images, however, several problems in terms of image quality have occurred as compared with the image quality obtained by a laser imager. An example of such problems will be described below.
FIG. 22 shows an example how a chest X-ray image is printed on a transparent film by this method. Reference numeral 100 denotes a film. According to a general X-ray photograph, on the shoulder portions, the density gradually changes. In this example, however, contours 101 appeared. Such a contour will be referred to as a pseudo contour to indicate that a contour appears on a portion on which no contour should appear. Such pseudo contours appeared on portions where the density gradually changed. If such pseudo contours appear, the quality of the image deteriorates, and image diagnosis as the essential purpose is adversely affected.
When the reasons for the occurrence of such pseudo contours were analyzed, the followings were found to be causes.
Consider a portion where the density gray level changes from 3043 to 2974 with reference to FIG. 19. On this portion, the combination indicated by No. 37 is mainly used first, and then the combination indicated by No. 36 is then mainly used. When the combinations indicated by Nos. 37 and 36 are compared, it is found that the types of inks used greatly differ; the inks C, D, E, and F are used according to No. 37, and the ink B is used according to No. 36.
Whether the combination indicated by No. 37 or the combination indicated by No. 36 is used is determined by the result of error diffusion processing. In general, on a portion where the gradation gradually changes, the combination does not necessarily switch to another combination gradually, but may switch abruptly.
Inks are mixed to obtain a density like the one indicated by Table 1. In practice, however, an error occurs. In addition, even if a correct value is obtained when inks are mixed, the value slightly changes with time due to evaporation or the like. According to experiments, a change of 2 to 3% can occur normally, and a change of about 5% may occur. If, for example, the density change is 3%, the transmission density becomes about 0.89xc3x970.03=0.027 in the case of the ink B. Assume that when the combination indicated by No. 37 switches to the combination indicated by No. 36, the inks C, D, E, and F have correct values. Even in this case, if the ink B changes 3%, an error of 0.027 occurs in terms of transmission density. If, for example, the above switching occurs in 50% pixels in a given small area, an error of 0.0135 occurs in this area in terms of average density.
The human eye density resolution with respect to a transparent film is 10 bits or more, which is 0.003 in terms of transparent density. That is, a portion where no contour should exist is recognized as a contour if there is a density difference of 0.003 or more in this portion.
The value xe2x80x9c0.0135xe2x80x9d as the above density error is sufficiently large as compared with this numerical value xe2x80x9c0.003xe2x80x9d. With this density error, a pseudo contour easily appears. In addition, nozzles vary in discharge amount. If the discharge amounts uniformly vary, since the number of nozzles is large, the overall variations become small owing to an averaging effect. If, however, the discharge amounts vary partially among chips, this also appears as a density error.
The following is another problem in terms of image quality.
According to the error diffusion scheme, if the minimum printing OD value is smaller than a desired OD value, no ink is ejected up to a certain threshold, and ink having the minimum printing OD value is ejected only when the threshold is reached. For this reason, on a portion where the density gradually increases from transparency, a phenomenon (sweep-out phenomenon) occurs, in which a transparent portion continues up to a certain point, and the density abruptly increases from this point.
In addition to the problems associated with image quality, a problem arises in error diffusion processing in terms of processing time. That is, a long processing time is required because calculation must be done for each pixel every time the image to be printed changes. When an error is to be diffused to neighboring pixels, in particular, many multiplications and divisions must be done. If the pixel size is reduced to 300 dpi or 600 dpi to obtain a high-resolution image, the processing time is prolonged due to a large number of pixels. In the case of medical images, in particular, since images are often printed on large-size films having a size of 14xc3x9717 inches, the above problem becomes serious.
The case where very high image quality is required has been described above. In some cases, however, high-speed printing may be required while the demand for image quality is not very high, and an inexpensive printer capable of printing high-quality may be required, which is realized by simplifying the mechanism.
The present invention has been made in consideration of the above problems, and has as its object to provide a printing method and apparatus which can easily change output characteristics in accordance with the purpose of a printed image and the like, and can print a high-quality image at high speed.
In order to solve the above problems and achieve the above object, according to the present invention, there is provided a printing apparatus for supplying printing materials from a printhead, which is used to supply a plurality of types of printing materials having the same color and different densities on the basis of density information values obtained from image data, onto each pixel to print an image including a plurality of pixels expressed by combinations of the plurality of types of printing materials on a printing medium, comprising a data storage unit storing data made to correspond to combination information representing a combination of the printing materials with respect to each address information representing a position of each of the plurality of pixels in each of the density information values, acquisition means for acquiring combination information of printing materials to be used to print a target pixel by looking up data stored in the data storage unit on the basis of a density information value and address information of the target pixel, and printing control means for printing the target pixel by supplying printing materials from the printhead onto the target pixel on the basis of the combination information acquired by the acquisition means.
In addition, there is provided a printing method of supplying printing materials from a printhead, which is used to supply a plurality of types of printing materials having the same color and different densities on the basis of density information values obtained from image data, onto each pixel to print an image including a plurality of pixels expressed by combinations of the plurality of types of printing materials on a printing medium, comprising the acquisition step of looking up a data storage unit storing data made to correspond to combination information representing a combination of the printing materials with respect to each address information representing a position of each of the plurality of pixels in each of the density information values, thereby acquiring combination information of printing materials to be used to print a target pixel on the basis of a density information value and address information of the target pixel, and the printing control step of printing the target pixel by supplying printing materials from the printhead onto the target pixel on the basis of the combination information acquired in the acquisition step.
Other objects and advantages besides those discussed above will be apparent to those skilled in the art from the description of the preferred embodiments of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate exemplary embodiments of the invention. Such examples, however, are not exhaustive of all the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.